Following advice from the independent Migration Advisory Committee, the Government introduced an annual limit on Tier 2, the skilled work route, in 2011. In 2012 the Government announced that from 6 April 2016 Tier 2 migrants who apply for settlement in the UK will be required to meet a minimum annual salary requirement of £35,000.
The Home Office published full impact assessments on these changes when the rules were laid before Parliament. The impact assessments are available on the gov.uk website as follows:
Annual limit: https://www.gov.uk/government/publications/migration-permanent-limit-tier-1-and-tier-2-impact-assessment
Under the annual limit, NHS trusts have been given more than 1,400 Tier 2 certificates of sponsorship for nurses since April this year but over 600 of the places allocated to them in April and May this year have been returned unused.